The invention relates to the field of linear motors and in particular to linear motors having a covered secondary member.
In general, a linear motor includes a, normally fixed, secondary or reactive member, having in case of a synchronous linear motor a plurality of magnetic elements (permanent magnets) arranged in side-by-side relationship to define a guideway for a rotor which is movable relative thereto. The rotor interacts with the magnets of the secondary member and converts this interaction in a translatory motion.
Such linear motors are used as drives in various technological fields, for example, in machine tools, machining devices such as automatic drills, automatic milling cutters, automatic lathes, automatic placement machines or automatic bonding machines, including vehicle drives in magnetic levitation trains, etc.
The structural length of the guideway depends on the intended use of the linear motor and should be at least as long as the required displacement path plus the length of the rotor. In order to have flexibility in connection with the design of linear motors of different length, the secondary members are made up of shorter secondary member elements, which are arranged, in side-by-side relationship until reaching the desired length.
The hereby created butt-joints are, however, disadvantageous in particular when exposed to rough whether, because dust, operating liquid, aggressive materials etc. can accumulate there. It also happens frequently that solid structures, tool pieces etc. migrate onto the guideway, for example, when drill bits break off from automatic drills. In the most adverse case, these solid materials may be snatched by the rotor and forced in the area of the butt-joints into the secondary member.
To avoid this, the guideway is, typically, provided with a cover, with the cover normally designed as a plastic film which is glued onto the guideway of the secondary member. Oftentimes, however, the butt-joints cannot be neutralized adequately, thereby causing unevenness that is favored of being exposed to the afore-mentioned mechanical stress; However, also the other regions can be damaged fairly easily. Thus, the plastic films must be replaced from time to time. As they are glued onto the guideways of the secondary member, replacement is relatively time-consuming and cost-intensive and causes shutdown periods of the machine.
DE 195 01 938 A1 shows a secondary member for a linear motor in which the magnetic elements are enclosed by a cover portion of, preferably, magnetic material. However, as the rotor of the linear motor is concerned here, the afore-stated problems as relating to long stators are not relevant here. Furthermore, the cover portion is connected with fastening means, namely screws, for the entire length of the rotor so that a simple exchange is not possible.
It is therefore an object of the invention to provide a secondary member for a linear motor and to provide a linear motor, with a cover reliably protecting the running surface of the secondary member and with the cover being easily replaceable.
In accordance with the invention, the cover of the secondary member has magnetizable material.
In general, it was assumed that no magnetic material should be introduced in the air gap of a linear motor, i.e. between secondary member and rotor, because the magnetic flux between rotor and secondary member is impaired. In contrast thereto, the invention has, however, recognized that this adverse affect can be exploited in a positive way. By being made of magnetizable material, the cover can be simply placed onto the surface of the secondary member and can adhere, without any need for additional means, solely by the action of the magnetic elements in the secondary member.
Hereby, it may be advantageous to provide additional fastening means at the ends of the secondary member track for anchoring upon the assembly base the cover part which projects beyond the ends of the secondary member track. On the one hand, the ends of the secondary member define particularly exposed attack areas so that an additional attachment is desirable, on the other hand, the ends may in this case also have non-magnetizable material. In accordance with the invention, no additional fastening means should be provided between the joint gaps of the secondary member, so that the cover is held between the ends of the secondary member and in particular in the area of the air gap substantially by the magnetic force only.
The cover may be made in one piece of magnetizable material, or a non-magnetic material can be provided in predetermined sections with magnetic areas, for example, strips which extend along the guideway and contact the surface of the secondary member. As the losses due to magnetic material are proportional to the layer thickness in the air gap, on the one hand, and proportional to the carrying capability, on the other hand, the magnetizable material should have a saturation induction of no more than 1.5 Tesla, at a layer thickness of preferably less than 0.5 mm. To realize a sufficient adhesion, a minimum saturation induction of 0.3 Tesla should be provided, and to ensure a sufficient function of the cover as protection against mechanical stress, a minimum layer thickness of 0.1 mm should be provided. A suitable material is, for example, special steel with a saturation magnetization of xc2xe to 1.5 Tesla.
Covers in the range of the afore-stated data adhere, on the one hand, sufficiently firm onto the secondary member, afford protection against mechanical damage and can be applied in one piece over the entire length of the guideway and, for example, delivered as roll material; the power losses are approximately in the range of 5 to 6% of the original thrust force.
As the material is sufficiently firm against mechanical stress and can be designed as a single-piece web for stable positioning on the secondary member, a preferred variation includes the application of a length scale onto the material, for example, by engraving, imprinting, bonding etc. Such length scales, which are necessary for the operation of a linear motor for position control, have been attached heretofore laterally on the secondary member and read by a reading head on the rotor. This lateral attachment has, however, the effect that a possible horizontal tilting of the rotor with respect to the secondary member may cause a measuring error in longitudinal direction of the scale. By attaching, in contrast thereto, the scale on the cover of the secondary member, preferably in the middle of the guideway, a tilting of the rotor leads only to a lateral shift of the reading head upon the scale but not to a shift in the longitudinal direction, so that the configuration according to the invention results further in a more accurate measurement of the position.
Details of the design of the scale are not described here as such scales, as explained, are known to the artisan.